In acute bronchitis, histamine is the primary mediator responsible for increased vascular permeability. It is released from mast cells upon irritation or infection, causing vasodilation and endothelial contraction. This leads to the leakage of fluid and inflammatory cells into the bronchial mucosa, resulting in edema and characteristic cough. Prostaglandins and bradykinin play secondary roles by modulating inflammation and pain, but histamine’s rapid action on blood vessels is the key factor in acute bronchial inflammation.

Reference: Robbins Basic Pathology, Kumar, Abbas & Aster, 10th Edition.